Aromatic carboxylic acids, such as terephthalic acid, isophthalic acid, and napthlene dicarboxylic acid are useful chemical compounds and are raw materials in the production of polyesters. In the instance of terephthalic acid, a single manufacturing facility can produce greater than 100,000 metric tons per annum as feedstock for a polyethylene terephthalate (PET) facility.
Terephthalic acid (TPA) can be produced by the high pressure, exothermic oxidation of a suitable aromatic feedstock such as para-xylene. Typically, these oxidations are carried out in a liquid phase using air or alternate sources of molecular oxygen in the presence of a metal catalyst or promotor compound(s). Methods for oxidizing para-xylene and other aromatic compounds such as m-xylene and dimethyinaphthalene are well known in the art. These oxidation reactions will typically produce reaction gases generally comprising oxidation reaction products such as carbon monoxide, carbon dioxide, and methyl bromide. Additionally, if air is used as the oxygen source, the reaction gases may also contain nitrogen and excess oxygen.
Most processes for the production of TPA also employ a low molecular weight carboxylic acid, such as acetic acid, as part of the reaction solvent. Additionally, some water is also present in the oxidation solvent as well as being formed as an oxidation by-product.
Oxidations of this type are generally highly exothermic, and although there are many ways to control the temperature of these reactions, a common and convenient method is to remove the heat by allowing a portion of the solvent to vaporize during the reaction. The combination of the reaction gases and the vaporized solvent is referred to as a gaseous mixture. The gaseous mixture contains a considerable amount of energy.
Because water is formed as an oxidation by-product, at least a portion of the gaseous mixture either as vapor or condensate is usually directed to a separation device, typically a distillation column, to separate the water from the primary solvent (e.g. acetic acid) so that the water concentration in the reactor is not allowed to build up.